1. Field of the Invention
This invention relates to a novel silver catalyst for producing ethylene oxide by the oxidation of ethylene with molecular oxygen, and a process for producing the catalyst.
2. Description of the Prior Art
A method known for preparing a silver catalyst for use in the oxidation of ethylene with molecular oxygen comprises impregnating a porous refractory carrier with an aqueous solution of a silver salt complexed with an amine (encompassing ammonia) and heating the impregnated carrier with air or the like to deposit silver on the carrier. By using amines, the silver salt decomposed and reduced at low temperatures is converted into an amine complex, and thus can be formed into a uniform aqueous solution. Hence, fine and uniform silver grains can be deposited on a porous carrier composed of a molded article of a refractory material to give a catalyst of excellent performance.
According to the description of Japanese Patent Publication No. 22146/1980, to deposit silver from a silver salt-containing aqueous solution and make a catalyst, heating at 100.degree. to 375.degree. C. for 2 to 8 hours is required, and air is used as a heating medium. Investigations of the present inventors have shown that the above method of heating results in a non-uniform distribution in the amount of deposited silver within a catalyst particle, and when the heating temperature and time described in the Examples of the above-cited Japanese Patent Publication are employed, the silver grains agglomerate and grow. For example, it has been ascertained that in a catalyst having 13% by weight of Ag deposited on a carrier having a surface area of 0.4 m.sup.2 /g which is prepared by calcination in air at 270.degree. C. for 2 hours, many of the deposited silver grains have a particle diameter of at least 0.4 micron, and thus, the silver grains are large and are distributed non-uniformly in each catalyst particle. For this reason, such a catalyst cannot exhibit a sufficient catalytic performance.